The active switching elements of integrated circuits are interconnected by metal lines deposited by various methods such as physical vapor deposition, chemical vapor deposition and evaporation. Typically, several levels of metal lines are used in an integrated circuit to allow crossovers. At certain locations, electrical contact is made between lines at different levels. Such locations are called vias. The drive of integrated circuits to submicron geometry results in vias and contacts of extreme aspect ratio and size. Filling submicron vias and contacts with a conducting metal is extremely difficult. However, if extreme care is not taken, voids will remain and the wafer will have to be discarded.
The use of aluminum as a material for filling interconnecting vias and contacts has been proposed. Surface diffusion is used to move sputter deposited aluminum into the via at temperatures of approximately 450.degree. C. This is disclosed, for example, in Armstrong U.S. Pat. No. 4,994,162. This uses a low temperature seed layer providing a continuous high quality diffusion path for subsequently deposited material to diffuse along. A high temperature low deposition rate step to allow efficient surface diffusion into the feature is employed followed by a high temperature high deposition rate step to complete the deposition.
The sputter deposition of collimated aluminum is well known as disclosed, for example, in Mikalesen et al U.S. Pat. No. 4,824,544, Homma et al U.S. Pat. No. 4,717,462 and Sakata et al U.S. Pat. No. 4,724,060. The Homma and Sakata references specifically employ collimation of the sputtered aluminum to fill vias having a relatively high aspect ratio.
Bulk diffusion as a mechanism for via filling has also been proposed by Sugano et al in the 1992 VMIC Conference Proceedings "Quarter Micron Hole Filling With SiN Sidewalls By Aluminum High Temperature Sputtering." With the Sugano process, the driving force for via filling is the interface between a titanium surface layer and the deposited aluminum. This process requires the presence of a continuous and high quality titanium surface layer on the sidewall of a via.
Tracy U.S. Pat. No. 4,970,176 discloses deposition of a relatively thick layer of aluminum at a first temperature and a subsequent deposition of a thin layer of aluminum at a higher temperature. The specification indicates that the temperature increase acts to reflow the aluminum through grain growth and recrystallization. As shown from the specification, the filling of the via starts from the bottom of the via and works up to the top. At the time the Tracy application was filed, typically the vias were of a size greater than one micron.
Sputter depositing aluminum at low temperatures improves step coverage reliability but at a drastic loss of overall step coverage. This is a result of insignificant aluminum mobility at temperatures less than 300.degree. C. The use of aluminum alloys with lower melting points can be used to lower the temperature necessary for significant mobility. However, there are several problems with using such alloys. Due to the low melting temperature of these alloys, they cannot withstand temperatures of subsequent processing steps. Germanium in aluminum-germanium alloys also tends to precipitate increasing resistance.